Current legislation on exhaust-gas emissions, which will become increasingly stringent in the future, makes high requirements in terms of the raw engine emissions and the after-treatment of the exhaust gas of internal combustion engines. In this context, the requirements for a greater reduction in fuel consumption and the further tightening of the emissions standards in terms of the permissible nitrogen oxide emissions (NOx emissions) pose a challenge to design engineers. In the case of gasoline engines, the exhaust gas is cleaned in a known manner by a three-way catalytic converter as well as by additional catalytic converters installed upstream and downstream from the three-way catalytic converter. Diesel engines currently make use of exhaust after-treatment systems having an oxidation catalytic converter or NOx storage catalytic converter, a catalytic converter for the selective catalytic reduction of nitrogen oxides (SCR catalytic converter) as well as a particulate filter to separate out soot particles, and if applicable, additional catalytic converters. In this context, ammonia is preferably used as the reducing agent. Since the handling of pure ammonia is laborious, vehicles normally employ a synthetic, aqueous urea solution that is mixed with the hot stream of exhaust gas in a mixing apparatus situated upstream from the SCR catalytic converter. This mixing process causes the aqueous urea solution to heat up, whereby the aqueous urea solution releases ammonia in the exhaust passage. A commercially available, aqueous urea solution generally consists of 32.5% urea and 67.5% water.
In view of the current and future legislation on exhaust-gas emissions, there is an increasing need for stricter adherence to the nitrogen oxide limit values, particularly in the case of motor vehicles with diesel engines. Exhaust after-treatment systems having an SCR catalytic converter or NOx storage catalytic converter located near the engine as well as a diesel particulate filter located near the engine are able to meet the current requirements, but such exhaust after-treatment systems are rapidly approaching their functional limits. Moreover, the further tightening of the emissions standards requires enlarging the volume of the catalytic converters and the volume of the particulate filter, reducing the exhaust-gas counter-pressure, and providing additional exhaust-gas sensors that serve to achieve improved regulation and to reduce the raw emissions of the internal combustion engine, and that serve for the onboard diagnosis of the exhaust after-treatment components. In order to analyze the exhaust gas, an additional NOx sensor can be installed downstream from the particulate filter, especially downstream from a differential pressure line for monitoring the particulate filter load. In this process, however, condensate droplets can be entrained by the exhaust-gas mass flow of the differential pressure line and, upon reaching the sensor, these droplets can cause a malfunction or even a complete failure of the sensor.
German patent application DE 10 2013 215 595 A1 discloses an exhaust system for an internal combustion engine, whereby a pressure sensor is provided on a particulate filter, whereby a pipe forms a connection line between the exhaust passage of the internal combustion engine and the pressure sensor. In this context, it is provided that this connection line is configured in such a way that condensation water flows out of the connection line into the exhaust passage in order to prevent the connection line from icing up at low temperatures.
International patent application WO 2008/042048 A1 discloses a method for thermal management in an exhaust system of an internal combustion engine. Here, the exhaust passage can have a line that connects the intercooler of the internal combustion engine to the exhaust passage. In this process, condensate that has been deposited on the intercooler can be utilized for the thermal management of the exhaust system.
A drawback of the prior-art approaches, however, is that water in the form of droplets can get into the exhaust passage and consequently, the problem of a potential malfunction or complete failure of a sensor in the exhaust passage, especially of a NOx sensor, caused by the dripping of water droplets is not solved.